Power amplifier



y 1932- E. T. BURTON 1,856,373

POWER AMPLIFIER Filed July 19, 1928 2 Sheets-Sheet l WV 25 E 23 g:-

INVENTOP EVERETT Z'BuRm/v ATTORNEY May 3, 1932.

E. T. BURTON POWER AMPLI FI ER Filed July 19, 1928 2 Sheets-Sheet 2 lNVENTOP EVE/PE T7 [BURTON ATTORNEY it is necessary that the Patented May 3, 1932 UNITED STATES PATENT OFFICE EVERETT T. BURTON, OF MILLBUBN, NEW JERSEY, ABBIGNOR TO BELL TELEPHONE LABORATORIES, INCORPORATED, OF NEW YORK, N. Y A CORPORATION OF YORK Application filed J'u'ly is,

This invention relates to electric wave repeating apparatus and particularly to the use signals by operating a vacuum tube amplifier circuit to its full power output whereby the normal ratio of the out ut impedance of the 10 tube and its connecte circuit is departed from durin portions of the time.

It is well nown in the art that if external impedance in the output circuit of a vacuum tube amplifier is greater than the plate re- 15 sistance, the characteristic of the amplifier becomes nearly linear when the amplifier is operated at low signal amplitudes, thereby rendering the output signal Waves practically distortionless. It is also generally known in 2 the art that this reduction in curvature of the characteristic may be enhanced by adding a series resistance in the output circuit of the vacuum tube. When it is desired to operate a vacuum tube at high power efiiciency, oad impedance be matched to the impedance of the tube and that any added resistance be very small or eliminated entirely. The characteristic of the circuit will become more and more curved 30 upto a definite limit as this resistance is reduced, thereby introducing considerable distortion in the output signal wave. Heretofore, in order to insure markedly increased power output ,without the accompanying distortion, amplifiers of the push-pull type were employed in conjunction with output transformers having balanced primary windings, but the use of these transformers is not always desirable, especially in circuits where the output currents are of low frequencies or high amplitudes. Even with the use of output transformers the connection of a pushpull amplifier to a line having the sides unbalanced with respect to ground is a problem involving considerable difiiculties.

According to the present invention the use of output transformers is avoided and high power efliciency in the vacuum tube amplifier is obtained when the platecircuit is worked 59 over practically the whole characteristic;

NEW'

rowan menu-Inn 1928. Serial No. 293,815.

The distortion thus introduced in the signal wave by the considerable decrease in the vacuum tube output circuit impedance is reduced by impressin a compensatin voltage on the grid circuit of the amplifier. The compensating voltage is produced by a second vacuum tube which has its rid circuit con nected in parallel and its p ate circuit connected in series with the grid circuit of the amplifier. The compensating tube may be arranged to correctfor the distortion inherent in the production of the even order multiple frequencies by operating on either the positive or negative portions of the output signal wave, thereby changing the plate current in-' put potential characteristic of the amplifier to be symmetrical on both sides of the zero operating point of the input voltage. Correction for both the odd and even order of multiple frequencies may also be effected by employing the compensating arrangement provided for the negative correction and adjusting the grid biasing batteries of theamplifier and the compensator tube so that the compensating voltage is applied to the input volt- .age over the entire utilized range of plate current, thereby giving to the amplifier a distortionless or straight line output over the entire range.

Characteristic features of this invention are, eliminating the distortion inherent in the production of even order multiple frequen-, cies in signal waves in the out ut of a high power vacuum tube amplifier circuit, wherein the output external 'impedance is not high in respect to the plate circuit impedance; causing the plate current-grid potential characteristic of the amplifier circuit to become symmetrical about the operating point of the normal grid potential by means of compensating voltages being su erposed on the inputvoltages; allowing the lament battery of the amplifier to be connected to earth if required and preventing the unbalance which would ordinaril introduce distortion in the output signals s ould an amplifier of the push-pull type be used on a grounded output circuit without an output transformer; eliminating the unbalancin efiects of capacity produced when the p -pull type of 1 I amplifier is used without an output transformer; allowing the use of an unbalanced form of output circuit instead of the balanced transformer type of ush-pull amph- 5 fier, while givin substantial y the same form of output as t e push-pull arran ement; and by causing the compensating vo tages to become effective at one extremity of the oper-' atingrange of plate currents rather than at 10 the middle in order to produce a'strai ht line characteristic over the entire utilized range of plate current. I

a The invention will be descrlbed 1n connecwhich:

Fig. 1 illustrates diagrammatically one embodiment thereof for efiectin' positive current compensation to correct ordistortion inherent in the production of even order tial characteristics of the power amplifier circuits shown in Figs. 1 and 3 when such circuits are designed to provide positive or negative compensation which may be utilized either over the entire operating range or over half the range;

Fig. 7 shows the circuit arrangement of Fig. 1 modified to se arate the input voltage, which, for examp e, may be retransmitted submarine cable signals, into its low and high frequency components so that these components may be compensated in different degrees and then impressed simultaneously on an amplifier; I

Fig. 8 shows another modification of Fig. 1, wherein compensation is efiected in the output circuit of the amplifier.

Referring to Fig. 1, the signal input voltage is received over conductors 11 and 12 froma generating or reproducing source 13, such as a transmitter, a repeater or an amplifier, and impressed on the grid circuit of a thermionic amplifier 14. Across the signal input circuit 11-12 and in shunt to the main stage amplifier 14 is connected a potentiometer 15, to which is adjustably connected the grid circuit of a compensator thermionic tube 16 so that both circuits are adapted to Q receive a portion of the signal input voltage.

tion with the accompanying drawings in The plate circuit of the main stage amplifier is connected directly to the transmisslon line 1-7 and the ground connection 18 and in order-to favor power efiiciency 1n the output circuit the plate circuit includes no added impedance between the plate and the trans-- mission line. The plate circuit of the compensator tube forms a art'of the grid cir cuit of amplifier 14. En this arrangement the voltage output of the compensator is added to the signal input voltage in the grid circuit of the amplifier to counteract for even order multiple frequencies appearing in the negative impulses. The normal or uncom-- pensated characteristic of the amplifier is 7 represented by the curve abc, which is indicative of high distortion in the positive current of the output signal wave of the amplifier. Therefore, in order to reduce this distortion and cause the degree of amplification to be approximately the same for both sides of the signal the plate circuit of the compensator tube is connected to the grid circuit of the amplifier by resistance 19, which forms 7 a part of both circuits. The C potential furnished by battery 20 in the compensator grid circuit is so adjusted that when no signal input voltage is applied across potentiometer 15 the compensator piate current is reduced to zero value. In a case where a negative signal is applied, the compensator tube grid is carried still farther negative with no compensating efiect on the amplifier because the compensator plate current remains at zero value causing thereby the amplifier plate current to follow the portion ba of the characteristic. When an increasing positive voltage is applied to the input the compensator plate will draw current which will increase with the input voltage. The voltage drop thus roduced across fisistance 19 issubtracted rom the positive'signal input, causing the plate current of the amplifier to follow the curve brie to thereby .form a new or compensated characteristic. If the compensator stage is properly adjusted the curve bde becomes an inverse image of the curve ba, thereby producing a characteristic which shows the positive output currents of the amplifier reduced to amplitudes symmetrical with those of the. negative output current. This type of compensator is referred ,to herein as a positive compensator since it afi'ects the positive mcoming signals.

In Fig.3 is shown an arrangement known as the negative compensator whereby negative current compensation is efiectedto correct for the type of distortion resulting in the production of even order multiple frequencies. In this arrangement the signal input volta e is received over conductors 11' and 12' rom a reproducing or generating source 13' and impressed on the grid circuit of the principal thermionic amplifier 14. The plate circuit of the amplifier, like that of the arrangement shown in Fig. 1, is connecteddirectly to transmission line 17 and to (ground connection 18 and includes no impe ance. Across the input circuit of the amplifier is connected potentiometer to which is adjustably connected the grid of a 15 compensator thermionic tube 16, so that only such of the signal input voltage is im ressed "on the compensator grid as require The compensator 16' is connected in the high side'of the signal voltage input circuit, be-

tween the potentiometer 15 and the grid of amplifier 14 the compensator being adjacent to the amplifier grid. Connected in the plate circuit of the compensator 16 and in the grid circuit of the amplifier is resistance 19', which like the resistance 19 in Fig. 1, serves to roduce a voltage drop across its opposite en s, but the voltage drop in this case is referring to Fig. 4, wherein a'b'c is the uncompensated plate current-input potential current characteristic of the amplifier 14.

' The C potential furnished by battery 20' in the compensator grid circuit is so adjusted that when no signal input voltage is applied across potentiometer 15' the compensator plate current is at zero value. In the case where a positive signal is applied to the conductors 11 and 12 the compensator grid is carried still further negative in respect to the compensator tubefilament with no compensating efi'ect on the amplifier because the compensator plate current remains at zero value, thereby causing the amplifier plate current to follow the portion b'c of the characteristic. When an increasing negative voltage is applied to the conductors 11-12 the compensator plate will draw cur rent which will increase with the input voltage. In response to the signals of negative current, the voltage drop produced across resistance 19' by the compensator plate draw ing current makes the amplifier grid more negative than it would be without compen- Therefore, this compensation is added to the signals of the negative current to correct for the distortion introduced in the amplifier plate current by the production of the even orders of harmonics, thereby causing the amplifier plate current to follow the curve bd' topform the new or compensated characteristic. I

Fig. 5 shows another arrangement for effecting negative compensation to correct for the distortion inherent in the production of even orders of multiple frequencies wherein the compensator tube '22 is connected between the low side of the input and the grid of the amplifier 23. The compensator grid is adj ustably connected to the potentiometer 24 and the compensator plate circuit is connect 'ed to the amplifier grid through resistance 25. In this arrangement the voltage drop across resistance 25 is added to the negative input voltage toproduce the compensated characteristic cbd shown in Fig. 4.

It may be noted in comparing the compensated characteristics shown in Figs. 2 and 4 that a positive compensator effectively reduces amplification of the amplifier stage,

while a negative compensator increases it. In this manner the characteristics are made substantially symmetrical on both sides of the zero operatlng oint of the input voltage,

which allows the circuit arrangement of Fig.

1 to use a higher input voltage for a given output amplitude and the circuit arrangements of Figs. 3 and 5 to require a lower input voltage for the same output amplitude.

It will be noted that the quality of the compensated unbalanced vacuum tube stage is essentially the same as that of a balanced or push-pull stage. However, in certain cases the unbalanced form may be used with greater advantage such as where it is desired to repeat signals to or between submarine cables. In such cases it would be necessary to pass ver low frequencies through the vacuumvtu e amplifier circuit which is grounded in the output circuit and may also be grounded in the input circuit. The low frequency and high output current values involved make transformer coupling of output impractical. vShould a push-pull stage be used in these cases the filaments and batteries thereof would be separated from the output ground by one plate circuit impedance and any leakage or capacity between the power supply and the ound connection would unbalance the circuit. The compensated forms of circuits shown invFigs. 1 and 2 have an earthed filament on each of the amplifiers and are not noticeably unbalanced by the capacity or leakage from the amplifier plate batteries. The input voltage to a push-pull stage must be equally divided between the two grids and must be free from ground, but in the compensated arrangements the input is not divided and in the case of the negative compensator, the inputcircuit may be connected to ground.

The principle of grid circuit compensation to correct for plate currentdistortionmay be generally applicable to unbalanced vacuum tube stages where good power efiiciency and high quality are required. In this connection it rnay be advantageous to allow the compensator to operate over the "entire range v of input rather than over only half the range,

as described above. This rinciple is illus-, trated in Fig. 6 where a" b c" is the uncompensated characteristic. Should the zero operating point of the input voltage be fixed ata point f slightly above the omt a" instead of at the mid-point of the c aracteristic as hereinbeforedescribed, the positive com- .pens'ator may be employed causing the compensatedcharacteristic to followthe straight line fe'fl' In thecase where the zero operat ing point was fixed at-di' the..v negative compensator would be used and the compensator characteristic would follow the straight line d"0".' Using a characteristic such as d"c" with the amplifier grid biased at a point sultsu In order to accurately-adjust the compensation to a given circuit a wide latitude of com ensator adjustments may be required.

In t e case of thevacuum tube cable transmitter, fair results were obtained by using a I single tube as'a compensator, but by arranging two compensatortubes in parallel practically perfect compensation over the entire range was obtainable. In Fig. 7 is shown a positive compensating circuit which has been successfully employed in transmission of submarine cable signals. Since the cable im-.

pedance is high to very low frequencies the rangement advantage is taken of the fact that less compensation may be effected onthe low plate current curvature is more. evident on the high signal-frequencies than on the low. Therefore,.Fig. 7 provides an arrangement whereby the high and low frequencies are introduced to the grid circuit of the amplifier through separate inputcircuits ina manner described in a plicants U, S. Piatent 1,763,880, iss'ued une 17, 1930. In th1s arfrequencies than on the high and this is accomplished by the use of a high resist'ancepo-' tentiometer placed acrossthe low frequency input and by utilizing any required portion of the voltage drop across this potentiometer for effecting compensation. In the circuit arrangement shown in Fig. 7 terminals 26 and 27 and28 and 29 are assumed to be con nected to the terminals of the low and the'high frequency transformers respectively, of the applicants copending application, supra, thereby providing separate inputs for the low and high frequency components of the signals; Both input voltages are impressed simultaneously on the grid of amplifier 30, which in accordance with the circuit actually employed, consists of twelve vacuum tubes arranged in parallel, the filament current for (1'', a good quality modulator or detector re-.

these vacuum' tubes together with that for the compensator tubes ing furnishedfrom an alternating current source 31.. Across the low frequency input terminals 26 and 27 and the high frequency input terminals 28 and 29are respectively connected potentiometers 32 and 33, the potentiometer 32 being of ah'i h resistance sees not to appreciably age than on the high frequency components.

In the compensator plate circuit which is common to both tubes 34 and 35 and which is included in the grid circuits of amplifier 30, resistance 36 serves to produce the voltage 'drop for effecting the compensation in the grid circuit in the same manner as resistance 19 1n Flg. 1 described above.

The other modification of Fig. 1 is shown in Fig 8 wherein compensation is efiected in the output circuit of the amplifier instead of 1n the input or grid circuit. The input voltage is impressed directly on the grid of am-- plifier 41 and the resulting plate current is impressed on the grid circuit of compensating tube 42 to cause a compensating voltage drop across the resistance 43 sufiicient to effect the necessary compensation. This arrangement produces the same effect on the plate currentgrid potential-characteristic of the amplifier as is shown in Fig. 2.

What is claimed is: 1. An amplifier system having a normally curved relation between input voltage or current and output voltage or current provided .with means for causing the relation to be straight or suitably mo fied over at least a part of the working range comprising a supplementary amplifier of curved characteristic, means for supplying thereto a portion of the input voltage or current, and means for supplying from said supplementary amplifier a suitably amplified voltage or current to a the circuit of said principal amplifier in a direction so that the curvatures of the two characteristics result in a modification of the curvature of the characteristic of the amplifier system; so that the distortion thereof is reduced.

2. In a signaling system comprising a space discharge amplifier'having an input and an output circuit for utilization of signals, signalproducingmeans connected to the input circuit of said amplifier, means for utilizing a part of the input voltage from said signal producing means to effect compensation in I the output of said amplifier to produce a sym*- metrical signal wave of positive and nega- A'tiveafirplitudes in said output circuit. 3. In a signaling system comprising a space superpose compensating voltages on a received impulse in the electrode circuitof said first named amplifier to thereby produce an outgoing signal wave of symmetrical positive 4 and negative amplitudes.

- 4. In a signaling system comprising a transmission line, a vacuum tube amplifier,

having its output circuit connected directly to said line, waveproducing means connected to the grid of said amplifier, a second vacuum tube having its grid circuit connected in parallel and its plate circuit connected in series with the grid circuit of said amplifier, a source of power for the filament of said amplifier, and means for connectingsaid source to ground without introducing distortion in the output waves. 7

. 5. Aconductive line system having one side of the line at ground potential, a space discharge tube amplifier havinglinput and output circuits, one of said circuits being connected to said line, the terminal of said circuit adjacent said line being grounded, in combination with the auxiliary discharge tube for supplying to said space discharge tube voltages derived from waves to be amplifiedv for modifying the characteristic of said space discharge tube.

6. An amplifier circuit, a space discharge device normally o erating on a curved portion of its amplification characteristic, .another space discharge device havinga curved characteristic, means for impressing a part of the input voltage of the first mentioned device upon the input circuit of the second mentioned device and means for. impressing the output voltage of the second mentioned device on the input" voltage of thefirst mention device in such a manner that the characteristic of said second mentioned device has a compensating eflect upon apart of said portion of the characteristic of said first mentioned device to impart a substantially symmetrical amplification characteristic to said am lifier circuit.

7. An amplifier-circuit comprising an amplifying device having at normal load an unsymmetricaLamplification characteristic, a rectifying device having a curved inputoutput characteristic, means for connecting the inputand the output circuits of said rectifving device in the input circuit of said amplifying device to increase the symmetry of the characteristic of said circuit by. combinergy directly from said signal producing means, said second amplifier being eiiectlve toing the characteristic of said rectifying device with that of said amplifying device.

8. An amplifier circuit for transmission oi a signal impulse, a first s ace discharge device included in said circuit having a curved; amplification characteristic, a second space discharge device having a curved amplification characteristic, means for impressing on.

said second device potential variations produced by said signal impulse to produce a distorted impulse, circuit means for impressing said distorted impulse upon said amplifier circuit .insuch a manner that the curved characteristics of said devices'are combined to reduce thecurvature of the amplification characteristic of said amplifier circuit.

In witness whereof, I hereunto subscribe my name this 16th day of July, 1928.

EVERETT T. BURTON. 

